This invention relates to scavenging oxygen. The present exemplary embodiments relate to a container. This invention finds particular application in conjunction with a container for food or beverages, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Polymers such as poly(ethylene terephthalate) (PET) are versatile materials that enjoy wide applicability as fibers, films, and three-dimensional structures. A particularly important application for polymers is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, polymers have some fundamental limitations that restrict their applicability. One such limitation is that all polymers exhibit some degree of permeability to oxygen. The ability of oxygen to permeate through polymers such as PET into the interior of the container is a significant issue, particularly for foods and beverages that are degraded by the presence of even small amounts of oxygen. For the purpose of this disclosure, permeable means diffusion of small molecules through a polymeric matrix by migrating past individual polymer chains, and is distinct from leakage, which is transport through macroscopic or microscopic holes in a container structure.
Besides food and beverages, other products affected by oxygen include many drugs and pharmaceuticals, as well as a number of chemicals and even electronics. In order to package these oxygen-sensitive products, brand owners have historically relied on the use of glass or metal packaging. More recently, brand owners have begun to package their products in plastic packages which incorporate either passive barriers to oxygen and/or oxygen scavengers. Generally, greater success has been achieved utilizing oxygen scavengers; however, oxygen scavenging materials heretofore have suffered from a number of issues. In particular, oxygen scavengers utilized to date rely on the incorporation of an oxidizable solid material into the package. Technologies utilized include oxidation of iron (incorporated either in sachets or in the container sidewall), oxidation of sodium bisulfite, or oxidation of an oxidizable polymer (particularly poly(butadiene) or m-xylylenediamine adipamide). All of these technologies suffer from slow rates of reaction, limited capacity, limited ability to trigger the scavenging reaction at the time of filling the container, haze formation in the package sidewall, and/or discoloration of the packaging material. These problems have limited the use of oxygen scavengers in general, and are especially significant for transparent plastic packaging (such as PET) and/or where recycling of the plastic is considered important.
It is well known that molecular oxygen will react with molecular hydrogen in the presence of a suitable catalyst. For example, Ann. Chim. Phys. Vol. 39, p 328 (1828) describes the platinum-catalyzed reaction between molecular hydrogen and molecular oxygen, and GB 1,188,170 describes the application of this technology to deoxygenating the contents of impermeable containers. In this patent, the container wall contains a redox catalyst and interior of the impermeable container is flushed with molecular hydrogen at the time of sealing. While this method is suitable for removing residual oxygen from the contents of impermeable containers, it would be of little value for permeable plastic containers. In the case of permeable plastic containers, any hydrogen introduced at the time of a sealing would be quickly lost because the permeability of plastics to hydrogen is quite high (for PET the permeability of hydrogen is about 15 times greater than the permeability of oxygen). In addition, oxygen would continue to permeate through the container walls and into the interior of the container over time. Hence any benefit from this approach would be short-lived, since any oxygen scavenged initially would be quickly replaced by ingressing oxygen, while any hydrogen initially present would be rapidly lost. Since brand owners generally desire product shelf-lifes in excess of three months (and sometimes as much as three years), such short-term protection is of little value.